I may be just going slightly mad, but hear me out.
I have a Macbook Pro running Catalina. My Magic Mouse 2 has been paired to it for a while.
I just installed Windows 10 to dual boot it using Boot Camp.
Now, I just got out the mouse thinking I needed to pair it again in Windows, but then I noticed it was already paired and working automatically! I really do not recall pairing it in Windows. I could be awfully forgetful and already did it, but I really do not remember doing so!
Did Apple make some feature in the Boot Camp tools that are installed in Windows causing bluetooth devices such as MM being synced between macOS and Windows?